For example, Patent Literature 1 identified below discloses a storage battery system including multiple storage batteries. Each of the storage batteries is configured by a secondary battery such as a lithium ion battery. The system discharges direct current power from the storage batteries, converts the discharged power by using a DC/DC converter, and increases voltage of the converted power. The system further converts the power into alternating current power by using a DC/AC inverter, and supplies the converted power to a load. In addition, the storage battery system receives power supplied from the outside, converts the received power by using the DC/DC converter, decreases voltage of the converted power, and then charges the storage batteries with the power.
Charge and discharge by each of the storage batteries is controlled by a controller provided for each of the storage batteries. In this case, each of the controllers performs control based on a command received from a higher controller. The higher controller is a device that adjusts demand power corresponding to power demanded by a load, and supply power corresponding to power supplied from the storage battery system to the load. Each of the controllers performs control in such a manner that the sum total of power to be charged or discharged by the storage batteries becomes equal to command power received from the higher controller. Accordingly, the storage battery group configured by the multiple storage batteries functions as a single large storage battery as viewed from the higher controller.
Charging rates of the respective storage batteries do not necessarily become uniform. Accordingly, generally adopted is discharge of the storage batteries in the descending order of the charging rates of the storage batteries at the time of discharge of the storage battery system, and charge of the storage batteries in the ascending order of the charging rates of the storage batteries at the time of charge of the storage battery system. This method securely achieves charge and discharge in correspondence with command power issued from the higher controller.